How to add details on a model
Those in the image definitely look modeled, not some sort of bump map or normal map trick, but both techniques can be used or even combined to achieve desired effect.

Deciding whether to model them or use some trick like normal maps or bump mapping/parallax mapping will largely depend on the purposed of your model , where or what for it will be used, and the limitations of that system.

If you are doing regular rendering with cycles/blender internal and have no/very little constraints in terms of polygon counts you should definitely consider modeling them for more realistic accurate results.

If your computer is slow or can't handle the details, or if they are not the main focus of the artwork and will be very far away/out of view you may consider using some sort of simplification like a bump map or a normal map to simulate the effect.

You can combine both techniques (modeling + normals) to obtain even further detailed model.

You may also choose displacement maps to generate actual geometry bumps from an image texture, at the expense of higher polygon counts. These are generally more suited for organic or irregular models.

If you are planning using it in some sort of game engine or real time visualization where complexity is a limiting factor you may bake the details into normal maps or bump maps from a high poly to a low poly version of the model

Either way, if you want to do it very accurately, you will probably have to model the gribbles in 3D yourself anyway, so you can use it later to bake it to the lower poly version.

As to the modeling part they seem to be simple boxes repeated across the surface, nothing too special about them. Use modifiers like Array for repetition, or the Shrink Wrap to place "2D" shapes across the larger surfaces then give them volume with a Solidify modifier. You can keep the greebles as a separate object parented to the main mesh so as to not increase the polygon counts/triangulation of the original, and still be able to turn them off or reuse them elsewhere with ease.